1. Field of the Invention
The present invention relates to a line in a display device which uses a current-driven element, such as, for example, an organic electroluminescence element (hereinafter referred to as “organic EL element”), as a display element in each pixel.
2. Description of the Related Art
Display devices which use current-driven organic electroluminescence (EL) elements as a display element in each pixel are known and, in particular, active matrix display devices in which a transistor (thin film transistor or “TFT”) is provided in each pixel for individually driving, for each pixel, the organic EL element provided in each pixel are now a focus of development.
FIG. 1 exemplifies an equivalent circuit corresponding to a pixel in an active matrix display device. A gate line GL is provided along a horizontal scan direction (row direction) of the display device and a data line DL and a power supply line PL are provided along a vertical scan direction (column direction) of the display device. Each pixel comprises a selection transistor Ts which is an n-channel TFT, a storage capacitor Cs, a p-channel element driving transistor Td, and an organic EL element EL. The selection transistor Ts has a drain connected to a common data line DL which supplies a data voltage to pixels positioned along the vertical scan direction, a gate connected to a gate line GL for selecting pixels positioned along the horizontal scan direction, and a source connected to a gate of the element driving transistor Td.
The element driving transistor Td is a p-channel TFT and has a source connected to the power supply line PL and a drain connected to an anode of the organic EL element EL. A cathode of the organic EL element EL is connected to a cathode power supply CV which is formed common to the pixels. One electrode of the storage capacitor Cs is connected between the gate of the element driving transistor Td and the source of the selection transistor Ts. The other electrode of the storage capacitor Cs is connected to a power supply of a constant voltage such as, for example, ground and a power supply line.
In this circuit, when the gate line GL is set to the H level, the selection transistor Ts is switched on, a data voltage on the data line DL is supplied via the selection transistor Ts to the gate of the element driving transistor Td, the element driving transistor Td allows a drive current corresponding to the gate voltage of the element driving transistor Td to flow from the power supply line PL through the element driving transistor Td, and light is emitted from the organic EL element EL at an intensity corresponding to the drive current. The data voltage on the data line DL is supplied to the storage capacitor Cs in addition to the element driving transistor Td and a voltage corresponding to the data voltage is stored in the storage capacitor Cs. Therefore, even when the gate line GL is set to an L level, the element driving transistor Td continues to supply the drive current according to the voltage stored in the storage capacitor Cs, and, thus, the organic EL element EL continues to emit light at an intensity corresponding to the drive current.
FIG. 2 is a plan view schematically showing an organic EL display device 100 disclosed in Japanese Patent Laid-Open Publication No. 2001-102169 (hereinafter referred to as “Reference 1”). In FIG. 2, the outermost solid line represents a transparent panel substrate 102 and a display region 104, shown by a dotted line and in which the above-described pixels are arranged in a matrix form, is positioned at a position slightly above the center of the panel substrate 102. A horizontal driver circuit 106 (hereinafter referred to as “H-related driver”) which is connected to the data line DL is formed along an upper side of the display region 104 and vertical driver circuits 108 (hereinafter referred to as “V-related driver”) which are connected to the gate lines GL are formed along the right and left sides of the display region 104. These drivers 106 and 108 comprise TFT or the like which is formed simultaneously with the TFTs provided in each pixel.
The thick solid line extending in the display region 104 along the vertical direction indicates the power supply line PL. Individual power supply line PL is connected to a wide portion 110 in the horizontal direction which extends along the lower side of the display region 104 and forms a comb shape as a whole. The wide portion 110 is further connected, near the center of the wide portion 110, to another wide portion 112 extending along the vertical direction. The wide portion 112 is connected to an input terminal T1 for the drive power supply placed at the lower side of the organic EL display device 110. Because the wide portion 112 in the vertical direction is connected to the wide portion 110 in the horizontal direction near the center of the wide portion 110, potential drops in the pixels near the left and right sides of the display region are balanced and the amount of potential drop can be reduced. In other words, variation in the potential among the pixels can be inhibited.
On the lower side of the organic EL display device 100, a plurality of terminals including a cathode terminal T2, a terminal T3 connected to the V-related driver 108, and a terminal T4 connected to the H-related driver 106 are placed in addition to the terminal T1.
In the organic EL display devices of the related art, the terminals for external connection are provided on the lower side of the panel substrate as described in the above-described Reference 1. There is, however, a demand that the terminals be placed on the right side or on the left side in relation to devices other than the display device. On the other hand, normally, because the demand for reducing the manufacturing cost is very strong, a change in layout on the panel substrate 100, such as the circuit structure and driver in the display region 104, is minimized. This is because the change of layout or the like may involve a change of masks which are used for forming the element and line and re-examination of the characteristics, which result in significant increase in cost. Therefore, when the input terminal for the drive power supply is placed at one end (left side) along the horizontal scan direction, for example, connecting the terminal and the portion of the wide portion extending along the horizontal direction in a minimum distance maybe considered. However, because all power supply lines PL are connected to the wide portion 110 and supply current to the EL elements in the pixels, when the terminal and the left side of the wide portion in the horizontal direction are connected, a large current flows through the wide portion, resulting in a larger potential drop towards the right side along the horizontal scan direction distanced from the terminal. Thus, the potential on the left side of the display region and the potential on the right side of the display region would significantly differ from each other. Such a difference in potential leads to a potential difference in corresponding power supply lines PL, resulting in different currents flowing through the organic EL elements depending on the position of the organic EL element on the panel, which is in turn recognized as a difference in the light emission intensity of the organic EL element and degradation of the display quality.